The present invention relates, in general, to field effect transistors, and more particularly, to a novel field effect transistor that has high transconductance.
Previously, the semiconductor industry produced P-channel field effect transistors (FETs) using both metal oxide semiconductor FET (MOSFET) technology as well as heterostructure FET (HFET) technology. For both technologies, the P-channel transistor performance was inferior to the N-channel transistor. This inferior performance appeared as a P-channel transistor that had lower transconductance, lower frequency response, and inferior pinch-off characteristics than a corresponding N-channel transistor. Precision analog and high speed digital circuits often were built using complementary structures that required both N-channel and P-channel transistors. The P-channel transistor's inferior performance limited the precision that could be obtained in complementary analog circuits. In digital circuits, the P-channel transistor resulted in high gate delays and lower operating frequencies.
Previous efforts of producing improved P-channel HFETs typically employed alloys of III-V semiconductor materials, such as indium gallium arsenide (InGaAs), as the channel material. In general, the alloy channel material was between adjacent layers of other III-V alloys such as aluminum gallium arsenide (AlGaAs). One problem of these previous HFETs was alloy scattering that occurred in the alloy channel material. As carriers traveled through the alloy, the carriers collided with the non-uniform lattice structure created by the alloy. These collisions decreased the carrier mobility and resulted in low HFET transconductance. Another disadvantage of these previous HFETs was low carrier confinement. The low mole fraction (typically less than 15 percent) concentration of small band gap material, such as indium, that could be included in these alloys was insufficient to provide a deep quantum well that confined carriers to the channel material. The primary result of the low carrier confinement was an inferior pinch-off characteristic. In addition to low confinement, the low mole fraction concentration of small band gap material also resulted in low mobility. To obtain a high mobility, the previous P-channel devices typically had to be operated at low temperatures (generally 77 degrees Kelvin or below).
Accordingly, it is desirable to have P-channel field effect transistors that have high transconductance, high mobility, high frequency response, and sharp pinch-off characteristics. It is also desirable that these P-channel HFETs have high mobility at room temperature, and that they do not suffer from alloy scattering.